1. Field of the Invention
The present invention relates to an electric machine having laminated cooling rings adjacent end turns of the stator core windings for transferring heat from the windings to the housing.
2. Background Art
The cooling of stator windings of electric machines is critical for providing satisfactory motor performance, reliability and durability. This issue becomes even more pressing when the motor is required to have a high power density (kW/m3) and low specific weight. An integrated starter/alternator is a good example where the electric motor is required to have high power density, low specific weight, and good cooling capability because of packaging constraints, fuel economy requirements, and the fact that the motor is in the engine compartment.
Thermally conductive epoxy potting materials have been applied on the end turn windings to improve the cooling of an electric motor. FIG. 1 shows a prior art motor 10 having such a feature. As shown, the motor 10 consists of a rotatable rotor 12 rotatably positioned within a stator core 14. The stator core 14 has first and second ends 16,18 with end turns 20,22 of the windings extending from the ends 16,18, respectively. A housing 24 having end plates 26,28 encloses the stator core, windings and rotor. A thermally conductive potting material 30 is positioned over each of the end turns 20,22 for carrying heat directly from the end turns 20,22 to the housing 24 for heat dissipation.
This potting technology provides limited value because the potting material has a fairly high thermal resistance, which limits the heat dissipation capacity. Additionally, the cooling potential of the potting material cannot be fully utilized because of gaps which are developed between the potting material and the winding, and between the potting material and the housing as a result of thermal cycling of the motor. The reason for the gap development is that the potting material, the winding and the housing have different thermal expansion coefficients. The gaps are filled with air, which provides minimal thermal conductivity.
Accordingly, it is desirable to provide an improved method and apparatus for cooling an electric machine.
The present invention improves upon the above-described prior art cooling method by providing two laminated aluminum ring assemblies at the opposing ends of the stator core which are embedded within the potting material. The laminated aluminum rings substantially enhance heat dissipation, while the laminations reduce eddy current buildup within the rings.
In a preferred embodiment, each lamination of the rings has a plurality of radially extending slots which provide air gaps to minimize eddy currents in a plane perpendicular to the central axis of the motor.
More specifically, the present invention provides an electric machine including a stator core having first and second ends and having windings therein with end turns of the windings protruding from the first and second ends of the stator core. A rotor is rotatably positioned within the stator core. First and second laminated aluminum ring assemblies are positioned against the first and second ends, respectively, of the stator core in contact with the housing. Thermally conductive potting material is positioned between the end turns and the respective first and second ring assembly at the first and second ends of the stator core, thereby creating heat dissipation paths from the end turns, through the potting material and the ring assemblies to the housing.
Accordingly, an object of the invention is to provide an improved method and apparatus for cooling an electric machine wherein laminated aluminum ring assemblies are provided at opposing ends of the stator core for dissipating heat through a potting material from the end turns of the windings.
The above object and other objects, features and advantages of the invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.